Pain abatement research is a major area of study which goes hand-in-hand with pain research itself. In many cases pain is a symptom of an underlying malady so the presence and nature of the pain in these cases is sometimes essential in aiding awareness and the diagnosis of the underlying illness. The abatement of pain has traditionally been effected using various external and internal treatments. Examples of external treatment include acupuncture, electro-shock treatment using transcutaneous electrical nerve stimulation (TENS), use of temperature such as application of hot or cold packs or topical application of cooling or heating formulations. Examples of internal, invasive treatments include drug treatments by oral administration or injection or injection of freezing agents. Where feasible, the external physical methods of alleviating pain are preferable over the invasive, internal techniques for obvious reasons.
The application of hot or cold to localized pain such as muscle or tendon pain to reduce swelling has a long history. There are many devices for heating or cooling parts of the body. Hot water bottles and ice or cold packs are among the oldest and simplest devices for applying heat and cooling respectively. Another type of device is the heating blanket that uses electrical resistive heaters for heating. U.S. Pat. No. 4,094,357 discloses a heat transfer blanket which uses heat pipes coupled to heating/cooling systems. U.S. Pat. No. 5,269,369 teaches a body suit which utilizes a system of heat pipes to redistribute body heat for heating or cooling the person wearing the suit.
U.S. Pat. Nos. 4,459,468 and 4,962,761 disclose fluid circulation systems for use with thermal bandages, pads or blankets. A drawback to these circulations systems is that they are quite bulky and awkward since they use large fluid pumps between the heat exchanger and the blanket or pad being heated or cooled. Other systems employ condensers, refrigerants and evaporator coils which are also bulky, awkward and of limited mobility.
The use of heating or cooling to alleviate certain types of pain by stimulating or constricting blood flow is well known. U.S. Pat. No. 5,314,456 issued to Cohen discloses a heating pad for relief of headache-related back, neck and head pain designed to induce venous constriction and enlargement to alter blood flow in the head to relieve headaches. Similar devices are disclosed in U.S. Pat. No. 4,061,898 directed to a heating cap for alleviating headaches and U.S. Pat. No. 4,781,193 which is directed to a headache treatment device which specifically heats the top of a patient's head using a resistively heated cap and cools a portion of the head around the patient's temples and brow using a cooled headband. A conventional refrigeration cycle uses a bulky and heavy compressor and condenser to cool the head band.
Temporal temperature modulation techniques have been developed wherein the applied temperature is switched between hot and cold. For example, U.S. Pat. No. 4,585,002 discloses a method and apparatus for alleviating pain through somatosensory stimulation achieved by frequent alternating temperature stimulation of the skin adjacent to the area of pain. This patent teaches that the habituation to heat and cold can be overcome by cycling one after the other to cause a dynamic temperature sensation more intense than achievable with either hot or cold individually. The frequency of the temperature change from hot to cold or vice versa is several times per minute which is observed to induce the maximal dynamic phase of the thermoreceptors in tissue. An improvement over U.S. Pat. No. 4,585,002 is disclosed in U.S. Pat. No. 4,860,748 which uses a thermal pattern generator to produce a variety of thermal wave patterns of hot and cold but with temperature switching frequencies of the order of a minute, greater than the switching times disclosed in '002. These devices use Peltier thermoelectric units so that the temperature depends on the polarity of the current through the thermoelectric unit.
It is believed that the mechanism of pain relief achieved through thermal somatosensory stimulation is related to the heating or cooling of the skin in the vicinity of the pain thereby swamping or overwhelming the pain stimuli. Briefly, it is believed that pain is a sensation that can be overwhelmed by other intense stimuli from the same or adjacent neural pathways. The brain responds to the intense stimulus by blocking it which has the beneficial effect of also blocking the sensation of pain. As disclosed in the publication "The Challenge of Pain" by Melzak et al., sensory modulation is believed to have a significant impact on the sensation of pain. Many forms of pain can be overwhelmed or obscured by other intense stimuli from the same or adjacent neural pathways wherein the brain responds to such intense stimuli by blocking the stimulus and concurrently the sensation of pain.
Heat and cold are among the most intense stimuli that can be safely applied to the human body within an appropriate temperature range without causing permanent injury or damage. Skin can withstand temperatures in the range from about 0.degree. C. to 45.degree.-50.degree. C. for extended periods of time without causing permanent damage or injury. While many prior art methods of alleviating pain teach application of heat or cold, these methods involve applying one or the other alone or alternating heat and cold temporally on the effected area. Application of heat or cold alone is accompanied by adaption which decreases the efficacy of the technique. In other words the maximum benefit occurs during the initial period of heating or cooling and drops off once the body adapts to the heat or cold.
A drawback to temporal temperature modulation techniques is that a significant fraction of the duty cycle is required for the skin temperature to reach the temperature of the device so the maximum sensory stimulus is not achieved. Further, among devices that employ heat and/or cold for somatosensory stimulation to block pain, adaptation or habituation to the sensation limits the effectiveness of the treatments. In all these cases the treatments are similar to naturally occurring phenomena and hence the body has built in methods of adaptation.
It would therefore be advantageous to provide a method of alleviating pain which provides a higher degree of somatosensory stimulation than taught by the prior art in which the limitations of adaptation are reduced.